Field of the Invention
This invention relates to communication systems which transmit information via radio waves from one point to the next. Specifically, the system comprises a transmitter or means which converts information signals such as audio or coded signals for propagation through or along a transmission medium. The transmitter is coupled to the medium and at least one receiver is coupled to the medium such that the information or modulated signal transmitted may be derived from the received modulated carrier wave signals and converted into signals corresponding to the information transmitted.
2. Description of the Prior Art.
In recent years, there has been a concerted effort on the part of federal and state highway agencies to improve emergency communication systems used by stranded motorists on rural and urban freeway systems, toll roads, and other limited access highways. Generally, contemporary emergency communication systems transmit coded information to a terminal station. The coded information is decoded at the terminal station and help is dispatched to the stranded motorist.
One type of contemporary emergency communication system consists of roadside call boxes positioned at specified distances along the perimeter of a limited access highway or toll road. A stranded motorist leaves his car, walks to the nearest box and places his request. The call box has means to encode the motorist's request and transmits the request in the form of a coded radio signal to a remote terminal station. On receipt of the coded radio signal, the terminal station decodes the signal and help is dispatched to the stranded motorist. In this type of emergency communication system, the terminal station does not generate an answer back signal acknowledging the receipt of the motorist's message. In other words, the motorist does not know whether or not his message has been received.
Although the above-identified type of emergency communication system, hereinafter called the fixed call box system, is accurate, if not precise, in locating the location of a stranded motorist, it has several drawbacks. One of the drawbacks is that the motorist has to walk across the highway or along the shoulder of the highway to operate the call box. The practice of crossing or walking along the shoulder of a highway in order to activate the call box places the stranded motorist in danger, in that he may be injured by automobiles traveling along the highway. In addition, the fixed call box systems have no indicating means to warn a motorist of failure in the system. The net result is that a motorist may be trying to obtain help from an inoperative call box. During an emergency, the lack of indicating means may be disastrous.
In addition, these systems are susceptible to pranksters. There have been several occasions where emergency personnel have been dispatched to call box locations only to find that there is no need for their services. The reason is that mischievous youngsters, traveling along limited access highways, generally stop their vehicle, activate the call boxes and then move on. Since the system has no way of determining the caller, the guilty party is not apprehended. Also, in a situation where emergency personnel is limited, a genuine call may go unattended.
In another type of emergency system, referred to as a mobile communication system, a call box is attached to a vehicle. In case of an emergency, the stranded motorist manually activates the call box and a coded signal is transmitted to a terminal station. In case of impact, some of these systems will automatically transmit. On receipt of this signal by the terminal station, an operator will determine the approximate location of the stranded motorist. The terminal station then transmits an "acknowledgement" to the stranded motorist informing him that his message has been received. Although the mobile communication system has solved some of the problems posed by the fixed call box communication system, the mobile communication system has several problems of its own.
Perhaps one of the greatest problems with the mobile emergency communication system is the inability of the system to determine the location of the stranded motorist accurately. In this type of emergency communication system, direction finding techniques are utilized to determine the direction from which the coded signal is received from the terminal station and hence the direction of the stranded motorist. Specifically, most of these systems utilize a so-called "Adcock" type antenna which operates on a nulling or peaking scheme to determine the azimuth or direction from whence the coded signal comes. In these types of systems, there is an ambiguity as to whether the signal is coming from the back or the front along the line of the azimuth relative to the position of a terminal station with an antenna. For example, suppose a terminal station with an Adcock type antenna is located between two parallel highways and a motorist is stranded on either of these highways. The motorist will activate the call box and the box will transmit emergency signals to the terminal station. On receipt of these signals by the Adcock antenna, an operator will determine the aximuth or direction of the stranded motorist, based upon the readings of the Adcock antenna relative to the terminal station. However, it should be noted that the azimuth crosses both highways and since the Adcock antenna is only capable of determining the azimuth from whence the distress signals come, there is no way for the operator to determine which highway the stranded motorist is on. In addition, this type of system does not identify the direction of travel of the motorist. It should also be noted that this type of system requires an operator to locate the approximate position of the stranded motorist.
Another drawback with the mobile emergency communication system is that both the call box and the transmitting antenna have to be mounted on the vehicle. In most cases, power for the system is obtained from the battery of the vehicle. In a typical situation, a motorist on entering a limited access road rents one of the systems and attaches it to his vehicle. On leaving the limited access road the motorist has to remove the system. The chore of attaching and removing the system is very cumbersome to motorists. Due to the cumbersomeness of the system, its usefulness for other purposes are rather limited, i.e., only motorists can use the system since the system has to be hard mounted onto a vehicle. Other prospective customers, for example, cyclist, and people who are hiking cannot use the system since it is impractical to mount. In addition, the system is expensive and consumes a relatively high amount of power.
Neither of the above described emergency communication systems meet the present day needs of motorists since the described systems are plagued with several inherent problems.
One of the pressing problems of the prior art emergency communication systems is that coded signals or messages are transmitted at higher power levels than is necessary to make contact with a terminal station and obtain help. The net result of high power transmission is that it aggravates the problem of electromagnetic interference within a shared frequency band, i.e., a frequency band which is assigned to a plurality of users. The problem of high power transmission stems from the fact that designers of prior art communication systems design for "worse case" conditions. The term "worse case" means that the designer will ascertain the maximum power which is required under the worse atmospheric conditions and will design the unit to radiate at fixed maximum power at all times to ensure contact with a base station.
Another problem of the prior art emergency communication systems is path loss. Path loss is the attenuation of a radio signal between finite points due to changes in atmospheric conditions due to rain, snow, fog, icing, time of day, month of the year, sun cycles, etc. The path losses also vary due to topography, ground electrical characteristics and other obstructions. Due to the uncertainty and unpredictability of path loss, the range (i.e., location) of a radio transmitter can not be determined accurately by the amplitude of the received signal. Instead of using amplitude (power) to determine range, the prior art systems determine the range (location) of a radio transmitter by measuring the time of arrival of a signal between two known points, or as it is called the "hyperbolic method". Another method is to measure "the round trip time" for a signal to reach a target and return or as it is called "active ranging." Notwithstanding the prior art ranging methods, the radio transmitter still has to transmit the signal at a relatively high power level (i.e., the maximum power required under the worse atmospheric conditions) to circumvent the effects of path loss, and as noted above, this is not desirable.